Waterborne latex coating compositions include an aqueous dispersion of particles of a film-forming polymeric resin and optional pigment particles which, when applied to a substrate and dried to form a film, impart substrate hide, water resistance, and durability to the film. Dispersants, surfactants, solvents and thickeners can be added to the coating compositions to adjust the viscosity of the composition, maintain a uniform and stable aqueous dispersion of the pigments and film-forming resin particles, and to improve properties of the paint film.
Waterborne latex coating compositions with higher volume solids provide more film build when the paint dries, which leads to improved hiding and improved scrub resistance. High quality paints tend to have higher levels of an acrylic latex, which binds the film together. The combination of high solids and high resin content makes these higher quality paints very responsive to water-soluble or emulsifiable associative thickeners such as, for example, polyurethane associative thickeners, often referred to as hydrophobically modified ethylene oxide urethane rheology modifiers (HEUR) thickeners.
Associative thickening involves non-specific interactions of hydrophobic end groups of a thickener molecule both with themselves and with components of the coating such as, for example, latex, pigments and dispersants. The thickener produces a reversible, dynamic network of thickener molecules and other components of the coating. The thickening effect is caused by interactions of the hydrophobic end groups of the thickener with other components of the formulation.
Some precomposite latexes, such as those available from DOW under the trade designation EVOQUE, include functional groups that allow several precomposite latex particles to adsorb on to a dispersed pigment particle such as, for example, TiO2, and act as spacers in the dry film to prevent flocculation of the individual TiO2 particles as the water evaporates from the wet paint. The use of precomposite lattices like the EVOQUE resins can lead to even higher viscosity paints before thickeners are added due to the increased hydrodynamic volume of the TiO2/EVOQUE latex composite.
To maintain KU viscosity in specification, low levels of HEUR thickeners are used, which can lead to bridge flocculated paint. Bridge flocculation occurs when a single thickener molecule is adsorbed on two different particles. This bridging effect can occur when there is more surface area available on the particles than associative species in the coating composition system (for example, thickeners, surfactants and dispersants). Bridge flocculated paints can show syneresis, poor flow and leveling.